Textinput: automatically shrinks

Fixes #2719.

I used the method we discussed in #2719. I assume that's clear; if not, please let me know so I can explain things. A few words on some decisions I made:

* I used `.css("height")` instead of `.height()`, as this might work faster. (That is because `.height()` might perform some logic because of the box-model. I've been working on this solution for a while and I've done the logic by thinking and trying, so there's no need to perform extra calculations on this.)
* I removed the min-height from textareas, because this gives trouble while setting the height to 0.
* I removed the height from textareas, it isn't used with this new solution.

The solution has been tested in Safari, Google Chrome, Opera, and Firefox.